The present invention relates to Telecommunications Digital Data Service (DDS) Transport Systems for digital signal transmission over telephone channels between transmission line elements, for example equipment used by a Network Service Provider to provide DDS to the Customer Premises and, specifically, to a two wire DDS Transport System using commercially available ISDN U-Interface Transceivers.
DDS was developed many years ago to utilize digital transmission techniques as contrasted to voice-band modems that carry the digital signals in analog form over telephone channels. This all-digital approach takes advantage of the regeneration, monitoring and protection applicable to digital signals. This provides a higher-quality service than can be realized using analog telephone systems for data communications. The Network Service Providers offered DDS to the customer""s premise equipment (CPE). DDS provides this high performance digital transmission of data using a four wire (two wire pairs) transmission medium. Two wires (one pair) are dedicated for transmitting data, while the other two wires (one pair) are dedicated for receiving data. DDS supports variable data rate that can range from 1.2 kb/s to 64 kb/s.
The Network Service Providers deliver DDS to the CPE by using an Office Channel Unit (OCU) and a Network Interface Unit (NIU). The OCU and NIU are typically located at the Network""s Central Office and Customer Premises, respectively. The OCU and NIU incorporates various options that allows the Network Service Providers the ability to remotely sectionalize problems, comply to the DDS tariffs and provide data rate adaption.
Prior art DDS is transmitted from the OCU 3 at the Network Provider""s Central Office 2 to the NIU 5 of the Customer""s Premises over two wire pairs 6a, 6b and 7a, 7b (four wires) (see FIG. 1). However, Network Service Providers are having a combination of the following problems: Copper wire shortagesxe2x80x94the cost of implementing four wires from the Network Service Provider""s Central Office to the Customer""s Premises is high. DDS Distance Limitationsxe2x80x94if the Customer is beyond the transmission operating distance of existing DDS equipment, the Network Service Providers must install a DDS Repeater to extend the DDS to the Customer. The use of DDS Repeaters are used on DDS transmission rates of 56 kb/s and 64 kb/s. The transmission operating distance is a function of the DDS transmission rate. The higher the DDS transmission rate, the shorter the DDS transmission operating distance. The cost of the DDS Repeater equipment, the DDS Repeater Mounting, and the DDS Repeater installation and maintenance is very high. Performance Limitationsxe2x80x94the present DDS transmission methodology involves the use of Alternate Mark Inversion line coding.
Accordingly, it is desirable to decrease the four wire delivery to a two wire delivery service. However, it is foreseen that attempts to do so would either limit the DDS transmission operating distance or require costly custom transceiver designs.
For example, there exists other non-ISDN transmission technologies such as DDS, traditional telephone service (Plain Old Telephone Service or POTS), Data (Ethernet, for example), or a combination of the these which can be transported over two wires using the ISDN U-Interface Transceivers. All of these technologies operate the ISDN U-Interface Transceivers at the ISDN line rate of 80 kbaud/s (data rate of 160 kbits/s). This 80 kbaud/s line rate is defined as an International Telecommunication standard for ISDN services at U-Interface demarcation point. The ISDN U-Interface Transceivers are manufactured to specifically operate at the line rate of 80 kbaud/s. Thus, prior art DDS transmission utilizing the ISDN U-Interface Transceivers at a line code rate of 80 kbaud/s does not operate at a far enough distance to eliminate the need to install DDS Repeaters. For this reason, DDS transmission is typically provided utilizing the four-wire system illustrated in FIG. 1.
Prior to the present invention, there existed only one effective two-wire DDS Transport System involving the OCU and NIU. This system, designed by Adtran, Inc. of Huntsville, Ala., utilizes a custom transceiver that uses an alternative line code called Simple Coded Pulse Amplitude Modulation (SCPAM). Such SCPAM transceivers utilize a custom chip set to produce the SCPAM line code and operate at delivery rates up to 64 kb/s. This system utilizes a termination unit which converts the SCPAM signal back to the industry standard AMI interface for delivery to the customer. However, the time and costs involved with the development of such custom transceivers are prohibitive.
The present invention solves the above identified problems by using commercially available ISDN U-Interface Transceivers to provide an effective two wire DDS (see FIG. 2). However, as stated above, ISDN U-Interface Transceivers are designed to transport ISDN service at 80 kbaud/s, which would have similar operating distances as existing four wire DDS at 56 kb/s or 64 kb/s transmission rates. Accordingly, to overcome these distance limitations, the present invention modifies these transceivers by scaling the operating frequency and incorporating a data buffer within the line elements, for example the OCU and NIU, to increase operating distance between the line elements.
The present invention relates to using ISDN U-Interface Transceivers into transmission line equipment such as OCU and NIU type equipment to provide a two wire DDS Transport System characterized by increased operating distance. The problem of using ISDN U-Interface Transceivers is in their operating distance limitation for DDS. To be an effective delivery system for DDS, it is very desirable to operate at longer distances than what the transceiver can normally provide. To overcome the distance limitation of using ISDN U-Interface Transceivers, the present invention scales the operating frequency of the transceiver and incorporates a data buffer within the transmission line equipment, such as the OCU and NIU to allow the use of U-Interface Transceivers to provide effective single channel (two wire) DDS transmission between transmission line elements.
Accordingly, it is the principle object of the present invention to provide for an effective two wire DDS transmission, without using a custom transceiver.
It is a further object of the present invention to provide for a cost-effective, two wire DDS transmission without distance limitations which would require DDS Repeaters.
It is another object of the present invention to provide for the use of ISDN U-Interface Transceivers in a two wire DDS Transport System characterized by increased operating distance.
It is also an object of the invention to provide a two wire DDS transport system comprising an ISDN U-Interface Transceiver having a scaled frequency and a data buffer within the transmission line elements.
Numerous other advantages and features of the invention will become readily apparent from the detailed description of the preferred embodiment of the invention, from the claims and from the accompanying drawings in which like numerals are employed to designate like parts throughout the same.